Method for determining the flank face contour of a gear skiving tool, gear skiving tool and use thereof

ABSTRACT

A clearance angle, of a blade-like tool or tool tooth of a tool for hob peeling workpieces is determined by defining the rake face contour of the tool and calculating the progression of path movement of the rake face during chip-breaking hob peeling, taking into account a pre-determinable transmission ratio between the tool and the workpiece determined by the respective number of teeth, and the desired tooth cross-section contour of the tool, and determining a tangential speed for points of the cutting edge of the tool during chip-breaking, wherein hob peeling is determined in the form of vectors that are displayed graphically as bundles for each point on the cutting-edge and a closed envelope surface is determined, which plus a desired clearance angle is selected as the shape for the flank face contour of the tool or of the flank face of the tool tooth. A tool is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a United States National Phase Application ofInternational Application PCT/DE2013/000653 filed Nov. 7, 2013 andclaims the benefit of priority under 35 U.S.C. § 119 of German PatentApplication DE 10 2012 022 439.7 filed Nov. 16, 2012, the entirecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for determining the flank facecontour, more particularly the clearance angle, of a blade-like tool ortool tooth of a tool for the gear skiving of workpieces, and also to atool for gear skiving, comprising a plurality of teeth, each havingfaces with cutting edges and, adjoining hereto, flank faces, which teethare arranged on a cylindrical or conical shell, wherein the tool can bedriven rotatably about a tool axis spaced a radial distance away fromthe workpiece axis and can be advanced into rolling engagement at acrossed-axes angle between the rotational axis of a driven workpiece andthe tool rotational axis, and finally to the use of said tool or of thetool produced according to the method.

BACKGROUND OF THE INVENTION

The first metal-cutting machining methods of the type stated in theintroduction are known from DE 243 514 C. The gear skiving uses as thetool a toothed wheel having end-face cutters. Unlike in slotting, thecutting motion is realized in that, via a skewed arrangement of the axisof the tool and the rotational axis of the workpiece, a cutting motionis generated by oppositely directed rotations of these parts. As itcirculates around the workpiece, the tool passes respectively throughtoothings which it cuts out of the workpiece.

In principle, a workpiece can be produced in gear skiving in a singlepass with just one performed feed motion. In the case of greatermaterial removal, however, several passes are sensible, in which thepeeling tool consecutively executes two feed motions with differentlylarge cutting depths, as is described, for instance, in DE 10 2008 037514 A1.

In order to improve the quality of the produced workpiece, in WO2012/098 002 A1, it is proposed that the workpiece-axis-parallelcomponents of the feed motion and of the cutting motion are directedoppositely to each other.

WO 2010/060733 A1 relates to a gear skiving apparatus in which anelectronic control device for positioning drives of the tool spindle andof the workpiece are provided, wherein the control device, in the toothcutting of a crudely toothed or untoothed blank, in the axial feed atthe end of the feed overlays a radial emergence motion from theworkpiece and/or at the start of the feed a radial immersion motion intothe workpiece.

Regardless of whether the gear skiving tool known according to the priorart has as the tool a cylindrical or a conical contour, fundamentallythe same rolling motions are obtained in the metal-cutting process, i.e.the tool operates with and without a face offset. However, due to thepath motion of the tool relative to the workpiece, at each moment of theengagement other clearance angles and rake angles are formed. In themost unfavorable case, during the cutter engagement rake angles of −50°or more can be formed, as a result of which the machining forces risestrongly, which ultimately, given inevitably arising vibration motions,can lead to not inconsiderable production inaccuracies. If the pathmotion is viewed in the reference system of the workpiece, then eachreference point of the cutter moves on a three-dimensional cycloid. Ifthe crossed-axes angle is neglected or an angle value of 0° is assumed,the trajectories in the external machining of a workpiece areepicycloids, and in the internal machining hypocycloids. Thetransmission ratio between the tool and the workpiece is decisivelyabove the number of rollovers of the tool until the same point isreached after a 360° passage.

In order to prevent one or more defective teeth of the tool leading tocorresponding defects in the finished workpiece, the number of teeth ofthe tool is chosen such that the number of teeth of the workpiece is anon-integral multiple. In the case of a non-integral multiple, thesituation would namely arise that the tool, as it circulates, alwaysmachines the same tooth space with the same tooth, so that geometricabnormalities of a “cutting tooth” of the tool cause correspondingworkpiece defects. Thus, a transmission ratio without a commondenominator or with a prime number is preferably chosen, i.e. forexample from 100 teeth of the workpiece to 29 teeth of the tool.

Given a positive crossed-axes angle, from flat cycloids evolve spatialroulettes, which can be used to analyze the motional paths of the facesof the tool. The kinematics of gear skiving turns out to be a complexmotion in which each cutter of a tooth of the tool immerses successivelyinto a tooth space of the workpiece and continues this radial motion ina rolling-down fashion as far as the tooth bottom, after which the toothcutter on the opposite wall of the tooth space is moved back out. Duringthe immersion and the withdrawal, the tool tooth cutter moves axiallyalong the workpiece tooth width. The rake angle changes constantly andcan even assume high negative values of up to −50°. At such highnegative rake angles, the tools are placed under extreme load by theincreasing cutting forces, which can give rise to considerable toolwear. Although the tools can be reground or exchanged for new tools,such works lead to downtimes in the production, which minimize theeffectiveness of the process. In the case of conical tools, there is theadded factor that the number of regrinding possibilities is limited dueto the cone.

SUMMARY OF THE INVENTION

Based on these insights, an object of the present invention is to definea method for determining the flank face contour of a gear skiving tool,a gear skiving tool, and a use thereof which allows a cutting operationwhich is gentler on the tools and offers a higher productivity rate.

According to the invention, a method is provided for determining theflank face contour, more particularly the clearance angle, of ablade-like tool or work tooth of a tool for the gear skiving ofworkpieces. The method includes a first step in which the face contourof the tool is defined and the progression of the path motion of theface of the tool during metal-cutting gear skiving is calculated, takinginto account a predefined or predefinable transmission ratio between thetool and the workpiece, which transmission ratio is determined by therespective number of teeth, and the desired tooth cross-sectionalcontour of the workpiece. In a second step, the tangential velocity ofeach point of the cutting edge of the tool during metal-cutting gearskiving is determined in the form of vectors, and these vectors arerepresented graphically as bundles at each point of the cutting edge anda closed envelope surface, within which no vectors lie, is determined.This envelope surface, plus the desired clearance angle, is chosen asthe shape for the tool flank face or flank face of a tool tooth.

According to another aspect of the invention, a tool for gear skiving isprovided comprising a plurality of teeth, each of the teeth having faceswith cutting edges and hereto adjoining flank faces. The teeth arearranged on a cylindrical or conical shell. The tool can be drivenrotatingly about a tool axis spaced a radial distance away from theworkpiece axis and can be advanced in rolling engagement at acrossed-axes angle between the rotational axis of a driven workpiece andthe tool rotational axis. A closed annular envelope surface, withinwhich no tangential velocity vectors lie and in relation to which theflank face of the respective tooth is inclined by 2° to 10°, preferably3° to 7°, is formed by the bundle of tangential velocity vectors of eachcutting edge point.

According to another aspect of the invention, a method is providedincluding using the tool produced according to the method by a toolguide, the cutting direction being opposite to the feed direction,without teeth of the workpiece having previously been machined withdirected feed motion in the cutting direction.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a view showing spatial motion of a face of a tool tooth;

FIG. 2 is a top view, parallel to the workpiece axis, of a spatialmotional curve according to FIG. 1;

FIG. 3 is a view of the path motion of the face of a tooth in theengagement region;

FIG. 4 is another view of the path motion of the face of a tooth in theengagement region;

FIG. 5 is another view of the path motion of the face of a tooth in theengagement region;

FIG. 6 is another view of the path motion of the face of a tooth in theengagement region;

FIG. 7 is a view showing an orientation-inverted velocity vector of eachcutting edge point of the face plotted at every point of the engagement

FIG. 8 is a view showing an orientation-inverted velocity vector of eachcutting edge point of the face plotted at every point of the engagement;

FIG. 9 is a view showing two tool engagements in a workpiece gaprepresented by the motional representatives; and

FIG. 10 is a view showing the corrected clearance surface and theclearance angle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is based on the following solution approach:

In FIG. 1, the spatial motion of the face 20 of a tool tooth isrepresented by way of example. In the chosen example, the tool has 19teeth and the workpiece 64 teeth in total, from which a transmissionratio of 64/19 is obtained. Of particular interest are the engagementregions 10, in which the tool engages cuttingly in the workpiece, i.e. aworkpiece tooth space. FIG. 2 shows a top view, parallel to theworkpiece axis, of the spatial motional curve according to FIG. 1, inwhich the engagement regions 10 respectively appear acute-angled. As canbe seen from the enlarged view according to FIGS. 3, 4, 5 and 6, whichshow various views of the path motions of the face of a tooth in theengagement region, the path curvature changes constantly during theimmersion and emergence of the tool into/from the workpiece tooth space.The face which is oriented approximately radially in the engagementregion cuts the workpiece at entry 11 into a tooth space at a positiverake angle, which, up to exit 12 from the tooth space, changes toward astrongly negative rake angle. In FIG. 6 is additionally recorded a pathmotion representative 13, which represents the motional path and thedirection of cut as a curved arrow. As can be seen, in particular, fromthe view in FIG. 7, which view is projected into a plane, the velocityvector which is assigned to each point of the active cutting edgebounding the face, and which consists of a scalar amount and themotional direction, changes in the course of passage through a toothspace. In FIG. 6 is additionally recorded a path motion representative13, which represents the motional path and direction of cut as a curvedarrow. For the present invention, the directional component of thevelocity vector is of particular importance. If, as represented in FIGS.7 and 8, the orientation-inverted velocity vector of each cutting edgepoint of the face is plotted at every point of the engagement, then abundle 21 of velocity vectors is acquired, which vectors form a closedenvelope surface 22 within which no velocity vectors lie. This envelopesurface, plus a desired clearance angle, which preferably lies between2° and 10°, determines the flank face contour of the tool tooth. In FIG.8, at a distance of about 5 mm from the face 20 is drawn a face 23,which—minus a marginal region 24 determined by the choice of clearanceangle—corresponds to the base of the cutting tooth of a tool. In otherwords, if the envelope surface 22 were chosen as the flank surface, theclearance angle would be 0°, which has to be correspondingly correctedto form a positive clearance angle.

The above-described tool is capable, on the basis of the chosenclearance angle, of immersing radially to a full depth or maximumadvance. After the immersion motion, the feed in the direction oppositeto the cutting motion can take place. In this way, the entry of the toolin the engagement zone can be utilized for the metal cutting. In thiszone, the effective rake angles are positive.

In FIG. 9, two tool engagements in a workpiece gap are represented bythe motional representatives 13 a and 13 b. If the feed takes place, asproposed in this invention, oppositely to the cutting motion, then 13 arepresents the first engagement and 13 b the second engagement in theworkpiece gap. The volume of metal removed is represented by the area14. The metal cutting is hence realized at the entry of the tool tooth,where the effective rake angles are positive, which is qualitativelyrepresented by the mean rake angle curve 15.

If the feed takes place in the same direction as the cutting motion, theexit zone of the tool tooth is used for the metal cutting and the metalcutting is realized at an effectively strongly negative rake angle,which results in high cutting forces within the process. In practice,the attainable qualities and tool lives are therefore limited.

The fundamental advantage of the invention lies in the use of thatsection of movement during the engagement in which the rake angles arepositive. Compared to the prior art, in this process management the chipcan slide more easily over the face and more heat is evacuated via thechip. Thus adhesion of the chip on the face is also lessened.

The above considerations apply, of course, also to blade-like tools,which are described, for instance, in DE 20 2011 050 054 U1.

The method according to the invention can be applied to determine theflank face contour in any tool whose number of teeth and number ofblades is preselected. The graphic representation of the path motion ofthe face of the tool tooth, as well as the determination of therespective tangential velocity of each cutting edge point, can be drawnup without great effort via a computing program, from which is obtainedan exactly contoured envelope curve, which, minus the desired clearanceangle, allows a geometrically clearly defined flank face to bedetermined. The computer-aided simulation of the path motions of a face,which in the simplest case can be of flat configuration, enables anoptimization of the flank face geometry and, at the same time, anoptimization of the machine-cutting process, in which the tool is guidedsuch that the immersion region of the tool tooth into the workpiecetooth space is utilized for the metal cutting. The tool can beconfigured as a monobloc, i.e. as a one-piece tool or as an assemblycomprising exchangeable tools (cutting inserts, blades). The toolpreferably consists of a tool steel produced by powder-metallurgicalmeans or of a hard metal; it works with and without a face offset andthe toothings to be produced can be internal and external toothings,straight and oblique toothings.

According to the workpiece material and the chosen cutting operation,the clearance angles lie within the range from 3° to 7° in order toprevent the chosen wedge angles from becoming too small, which wouldincrease the fragility of the cutting edges.

Preferably, the absolute rake angle lies between +10° and −30°, wherein,as a result of the tool setting angles in the metal cutting, at leastupon immersion into a tooth space of the workpiece, the effective rakeangle is positive.

The tool for gear skiving, comprising a plurality of teeth, each havingfaces with cutting edges and hereto adjoining flank faces, which teethare arranged on a cylindrical or conical shell, wherein the tool can bedriven rotatingly about a tool axis spaced a radial distance away fromthe workpiece axis and can be advanced in rolling engagement at acrossed-axes angle between the rotational axis of a driven workpiece andthe tool rotational axis, is designed such that a closed annularenvelope surface, within which no tangential velocity vectors lie and inrelation to which the flank face of the respective tooth is inclined by2° to 10°, preferably 3° to 7°, is formed by the bundle of tangentialvelocity vectors of each cutting edge point. The number of teeth isdetermined by the transmission ratio.

Preferably, said tool is used in such a way that the cutting directionis opposite to the feed direction, without teeth of the workpiece havingpreviously been machined with a feed motion directed in the cuttingdirection. As a result of the design of the flank face, the tool, in themetal cutting process, can be advanced to the full depth of theworkpiece tooth and withdrawn. In this way, the entry zone of the tooltooth is utilized for the metal cutting. In this zone, effective rakeangles are positive, and thus the metal cutting forces are lower than inmetal cutting using the exit zone. The metal cutting forces, and thusthe excitations in the process, are thus reduced to a minimum.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

The invention claimed is:
 1. A method for determining the flank facecontour, including a clearance angle, of a blade-like tool or work toothof a tool for the gear skiving of workpieces, the method comprising:defining the contour of the face of the tool, the face having cuttingedges and calculating the progression of the path motion of the face ofthe tool during metal-cutting gear skiving relative to a rotatingworkpiece, wherein the relative motion is defined by a predefined orpredefinable transmission ratio between the tool and the workpiece and acrossed-axis angle between a rotational axis of the workpiece and arotational axis of the tool, which transmission ratio is determined by arespective number of teeth, and a desired tooth cross-sectional contourof the workpiece; determining a tangential velocity of each point of acutting edge of the tool during metal-cutting gear skiving relative tothe workpiece in the form of vectors, and representing the vectorsgraphically as bundles of lines in a direction opposite to the directionof motion at each point of the cutting edge and determining a closedenvelope surface, within which no lines lie; and choosing the envelopesurface, plus a desired clearance angle as the shape for the tool flankface or flank face of a tool tooth.
 2. The method as claimed in claim 1,wherein the clearance angle is chosen to be between 2° and 10°.
 3. Themethod as claimed in claim 1, wherein a chosen absolute rake angle liesbetween +10° and −30°.
 4. The method as claimed in claim 1, wherein theclearance angle is chosen to be between 3° and 7°.
 5. The method asclaimed in claim 1, wherein, as a result of tool setting angles in themetal cutting, at least upon immersion into a tooth space of theworkpiece, a positive effective rake angle is chosen.
 6. A method fordetermining the flank face contour, including a clearance angle, of ablade-like tool or work tooth of a tool for the gear skiving ofworkpieces, the method comprising: defining the contour of the face ofthe tool, the face having cutting edges, and calculating the progressionof the path motion of the face of the tool during metal-cutting gearskiving relative to a rotating workpiece, wherein the relative motion isdefined by a predefined or predefinable transmission ratio between thetool and the workpiece and a crossed-axis angle between a rotationalaxis of the workpiece and a rotational axis of the tool, whichtransmission ratio is determined by a respective number of teeth, and adesired tooth cross-sectional contour of the workpiece; determining atangential velocity of each point of a cutting edge of the tool duringmetal-cutting gear skiving relative to the workpiece in the form ofvectors, and representing the vectors graphically as bundles of lines ina direction opposite to the direction of motion at each point of thecutting edge and determining a closed envelope surface, within which nolines lie; choosing the envelope surface, plus a desired clearance angleas the shape for the tool flank face or flank face of a tool tooth; andproducing a tool with the determined flank face contour including theclearance angle to provide the blade-like tool or tool with work teethfor the gear skiving of workpieces, and providing the tool with a toolguide, cutting direction opposite the feed direction, wherein teeth ofthe workpiece are machined in the cutting direction opposite to the feeddirection.
 7. A method for determining a flank face contour, including aclearance angle, of a tool or work tooth of a tool for gear skiving ofworkpieces, the method comprising: defining a tool face contour of aflank face of a tool structure, the tool face contour comprising cuttingedges; determining a transmission ratio between the tool structure and arotating workpiece structure and a crossed-axis angle between arotational axis of the workpiece and a rotational axis of the toolstructure based on a number of teeth and a desired tooth cross-sectioncontour of the workpiece structure; calculating motion of the flank faceof the tool structure relative to the workpiece during a metal-cuttinggear skiving process based on at least the transmission ratio;determining a vector of each point of a cutting edge of the toolstructure to provide a number of vectors, each of the vectorscorresponding to a tangential velocity of each point of the cutting edgeof the tool structure during the metal-cutting gear skiving processrelative to the workpiece, wherein the vectors are displayed graphicallyas bundles of lines in a direction opposite to a direction of motion ateach point of the cutting edge; determining a closed envelope surface,wherein no lines are provided in the closed envelope surface; andchoosing the envelope surface and a desired clearance angle as a shapefor the flank face of the tool structure or the flank face of a tooltooth of the tool structure.
 8. The method as claimed in claim 7,wherein the clearance angle is chosen to be between 2° and 10°.
 9. Themethod as claimed in claim 7, wherein a chosen absolute rake angle liesbetween +10° and −30°.
 10. A method according to claim 7, furthercomprising: producing a tool with the determined flank face contourincluding the clearance angle to provide the blade-like tool or toolwith work teeth for the gear skiving of workpieces, and providing thetool with a tool guide, cutting direction opposite the feed direction,wherein teeth of the workpiece are machined in the cutting directionopposite to the feed direction.
 11. The method as claimed in claim 7,wherein the clearance angle is chosen to be between 3° and 7°.
 12. Themethod as claimed in claim 7, wherein, as a result of tool settingangles in the metal cutting, at least upon immersion into a tooth spaceof the workpiece, a positive effective rake angle is chosen.